High-strength steel sheet having excellent workability

Abstract

According to the present invention, there is provided a high strength steel sheet, which has, for example, a tensile strength of 590 to 980 MPa or more, which has favorable workability, and which is useful for an automobile, etc. The high strength steel sheet of the present invention comprises 0.03 to 0.20% C (% by mass in chemical compositions; hereafter, the same holds true), 0.50 to 2.5% Si, 0.50 to 2.5% Mn, and further, preferably 0.02 to 0.2% Mo. Moreover, its metal structure includes ferrite and low temperature transformation phase. The mean grain size of the low temperature transformation phase is 3.0 μm or less. Further, grains whose size is 3.0 μm or less occupy 50% or more by area ratio of the low temperature transformation phase, and an average aspect ratio of the low temperature transformation phase is 0.35 or more.

The invention claimed is: 
     
       1. A high strength steel sheet comprising
 C: 0.03 to 0.20% (% by mass in chemical compositions; hereafter, the same holds true), 
 Si: 0.50 to 2.5%, and 
 Mn: 0.50 to 2.5%, 
 wherein a metal structure includes ferrite and low temperature transformation phase, a mean grain size of the low temperature transformation phase is 3.0 μm or less, grains having a grain size of 3.0 μm or less occupy 50% or more by area ratio of the low temperature transformation phase, and an average aspect ratio of the low temperature transformation phase is 0.35 or more, 
 wherein the steel sheet has a tensile strength of 780 MPa or more, 
 wherein the steel sheet is a cold rolled steel sheet, and 
 wherein the low temperature transformation phase contains martensite, bainite and pseudo pearlite. 
 
     
     
       2. A high strength steel sheet according to  claim 1 , further containing 0.02 to 0.2% Mo. 
     
     
       3. A high strength steel sheet according to  claim 1 , further comprising at least one element selected from the group consisting of
 Ti: 0.01 to 0.15%, 
 Nb: 0.01 to 0.15%, 
 Cr: 0.01 to 0.5%, and 
 V: 0.001 to 0.15%. 
 
     
     
       4. A high strength steel sheet according to  claim 1 , which contains 0.05% to 0.17% of C. 
     
     
       5. A high strength steel sheet according to  claim 1 , which contains 0.7 to 1.8% of Si. 
     
     
       6. A high strength steel sheet according to  claim 1 , which contains 0.50% to 2.3% of Mn. 
     
     
       7. A high strength steel sheet according to  claim 1 , wherein the low temperature transformation phase contains martensite occupying 10% to 80% by area ratio. 
     
     
       8. A high strength steel sheet according to  claim 1 , wherein the low temperature transformation phase contains martensite occupying 20% to 70% by area ratio. 
     
     
       9. A high strength steel sheet according to  claim 1 , wherein the low temperature transition phase contains martensite in 90% or more by area ratio. 
     
     
       10. A high strength steel sheet according to  claim 1 , wherein the mean grain size of the low temperature transformation phase is 2.5 μm or less. 
     
     
       11. A high strength steel sheet according to  claim 1 , wherein grains having a grain size of 3.0 μm or less occupy 65% or more by area ratio of the low temperature transformation phase 
     
     
       12. A high strength steel sheet according to  claim 1 , wherein the average aspect ratio of the low temperature transformation phase is 0.45 or more. 
     
     
       13. A high strength steel sheet according to  claim 1 , wherein the average aspect ratio of the low temperature transformation phase is 0.55 or more. 
     
     
       14. A high strength steel sheet according to  claim 1 , which has a tensile strength of 1180 MPa or more. 
     
     
       15. A high strength steel sheet according to  claim 1 , which has a tensile strength of 980 MPa or more.